1. Field of the Invention
The present invention relates generally to a magneto-optical recording medium and, more particularly, to a recording medium having both a RAM part recordable, reproducible and erasable by a recording/reproducing apparatus of a user and a ROM part reproducible but not rewritable by such an apparatus.
2. Description of the Related Art
The problem of protecting the copyright of computer software is currently becoming important. From the standpoint to provide such protected software, a variety of measures have been adopted on media in an attempt to prevent illegal use thereof with unauthorized duplication, and the recent trend generally seems that the software is provided in the form of ROM. In a CD-ROM which is an optical recording medium employed frequently of late due to its advantage that a large amount of information is recordable, the entire plane thereof serves as a read-only area, and discrimination between a regularly distributed medium and an illegally exploited one can be achieved merely by deciding whether writing is possible or not.
However, as viewed from the user's standpoint, a CD-ROM is disadvantageous because of its insufficient degree of freedom in practical use due to slow reproduction and nonexistence of a recordable/erasable user area. For this reason, there is currently proposed, in place of the above-described medium, a partial ROM which is a magneto-optical recording medium having both a reproduction-only area and recording/reproduction area. In the conventional partial ROM medium known heretofore, indented pits (ROM pits) are previously formed in a part of a substrate to define an area where changes of the reflectivity are applied as reproduced signals, whereby a partial ROM is achieved. According to this method, provision of read-only software is rendered possible by the technique substantially equal to that for the CD-ROM.
Since a substrate of an optical disk is produced by a method of injection-molding a resinous material, a metallic stamper is first required to serve as a master in manufacture of a conventional partial ROM medium. Therefore, in providing new software, it is necessary to produce a stamper where new information has been recorded. Since highly advanced apparatus and techniques are required for producing such a stamper, any software provider having none of the adequate equipment needs to request production of a stamper and a substrate from a third party when providing a conventional partial ROM medium. And there exists another disadvantage that, in case any inconvenience is found in a stage after start of mass production, it has been impossible heretofore to debug such inconvenience in the conventional partial ROM medium. Also for prevention of copying or duplication of software, it may occasionally be desired to sell a medium with some specific information (such as manufacture number, serial number and so forth) readable by a recording/reproducing apparatus of a user. However, in the conventional partial ROM medium, it has been impossible heretofore to meet such requirements.
On an ordinary magneto-optical disk medium, a plurality of grooves and a plurality of lands are formed alternately, and the land disposed between the adjacent grooves is used as a recording track where record marks are formed by magnetization in different directions conforming with data. The recording region is divided into a plurality of sectors, and an ID signal is preformatted with pits at the top of each sector where data is recorded. The ID signal is detected from a change in the amount of the reflected light, and the data is detected from a change in the polarization angle of the reflected light obtained by the magnetic Kerr effect. However, in such a land recording system, there exists a problem that the track density cannot be raised since none of the grooves is used as a data recording area. For this reason, there is proposed a land-groove recording system which uses each groove as a recording track as well.
According to the land-groove recording system, it is possible to increase the track density, but due to the structure where the recording tracks are mutually proximate, there occurs a phenomenon termed crosstalk as the data signal on the adjacent recording track is mixed into the reproduced signal. However, such crosstalk of magneto-optically recorded data can be eliminated by the mask effect based on magnetically induced superresolution which is capable of reproducing any record mark smaller in diameter than the spot of a reproducing laser beam, as disclosed in, e.g., Japanese Patent Laid-open Nos. Hei 3 (1991)-88156, Hei 5 (1993)-81717 and Hei 5 (1993)-342670. On the land-groove recording medium, it is necessary to preformat pits in the grooves as well as in the lands. However, the magnetically induced superresolution is responsive to the signal which is represented by a change in the amount of the reflected light from the pits. As a result, crosstalk relative to such signal raises a problem because reproduction of the ID signal is impossible.